1. Field of the Invention
The present invention relates to the storage of information by optical means, as well as to the reading of this information.
In technologies for the optical storage of information, the maximum density of storage is limited by the diffraction at the wavelength of the writing or of the reading. For the optical wavelengths commercially available in the form of laser diodes, this limit may be, for example, in the region of one bit per square micrometer. With the new blue laser sources emitting towards 0.4-0.5 .mu.m, it becomes possible to increase the storage of the information accordingly. Typically, an increase in storage density by a factor of 4 to 5 is expected as compared with current sources emitting in the near infra-red range. However, even these storage systems do not appear to be likely to enable surface densities of information suited to the storage of high definition television signals of a duration sufficient for large-scale consumer applications.
In this context, systems based on storage in volume have been proposed more recently. These systems should provide a gain by a factor of several tens in terms of surface density of information elements, but with the drawback of substantially increased complexity.
This is why the present invention proposes a new method of optical writing capable of giving rise to information carriers with high density of surface storage. The present invention also proposes a method for the reading of an information carrier with high density of surface storage.
2. Description of the Prior Art
At present, the approaches that enable surface storage are limited by problems of diffraction.
Indeed, when a writing laser beam is focused on the surface of a carrier, it is not focused at one point but in a region in which the distribution of density of power is the one shown in FIG. 1a. It is defined by the following surface equation: EQU D(x,y)=[2J1(Z)].sup.2 /Z.sup.2
with Z=2 EQU (x.sup.2 +y.sup.2).sup.1/2 (Sin A)/L
where
J1 is the first order Bessel function PA1 x and Y are surface coordinates, and PA1 L is the wavelength of the incident beam. PA1 it uses a laser with a wavelength L focused on the surface of the material (Ma) that is highly non-linear optically; PA1 the maximum power density of the focused laser beam is slightly greater than the threshold power density of the material (Ma) beyond which the material (Ma) can get transformed optically so as to record an information element that is appreciably smaller than the focusing spot of the beam used. PA1 only the regions (Ra) of the material (Ma) in which information elements have been recorded are transparent to the reading wavelength L1; PA1 the focusing of the reading beam at the wavelength L1 is done on the surface of the regions (Rb) of the material (Mb) that are facing the regions (Ra); PA1 the reading laser beam has a power such that the power-density at the center of the beam exceeds the threshold power solely in a small zone with a surface area that is several times smaller than the surface area of the focusing spot of the laser beam.
and
where
A is the angle defining the focus of the lens used for the convergence of the beam (this angle is shown in FIG. 1b). It is the angle of aperture of the focusing lens.
Thus the focusing spot, called the Airy spot, has a base is defined by a radius R.sub.o with EQU R.sub.o =1.22 L/2 sin A
These relationships are valid in the case of a uniform illumination of the pupil of the objective and for a less chromatic radiation. In practice, since the laser beam has a Gaussian distribution, the illumination in the pupil of the objective has an intensity with the shape of a truncated Gaussian curve. Furthermore, it has a certain spectral width. The distribution of illumination at the focus of the objective is not exactly an Airy function, but the approximation made herein is considered to be representative of the real phenomenon.
In the extreme case of a maximum focusing aperture (A=90.degree.), R.sub.om =1.22 L/2. However, for a smaller aperture, hence for a smaller field depth, the base of the Airy spot has a radius greater than R.sub.om. The radius R.sub.om corresponds rather to the size of the spot at mid-height of the curve D(x,y) and enables the definition of the size of an information element recorded with a sufficient density of power. Thus, typically, in using a recording beam focused at the wavelength L, it is not possible to record information elements having a lateral size smaller than L.
The present invention proposes the use of a material that is highly non-linear optically (Ma) and a focused laser beam having a maximum power density Pmax such that the threshold power density Pthreshold of the material (Ma) is slightly lower than Pmax. FIG. 2 shows that the invention uses the upper part of the curve D(x,y). It is concerned with the regions in which a small variation of the density of recording power leads to corresponding reductions at the level of the surface of the recorded information elements. The optical response of the material (Ma) should therefore be as highly non-linear as possible and the densities of power P.sub.max and P.sub.threshold are matched in such a way as to be on either side of the bending point of the curve illustrating the optical response of the material (Ma) with the density of power that it receives. This curve is shown schematically in FIG. 3. The hatched part of FIG. 2 corresponds to zones in which the material is transformed optically and which define the regions in which there has been a recording of information elements.
The size of the information elements may thus be far smaller than in the prior art and, hence, for an equivalent storage surface area, the storage density is notably increased.